FIG. 7 shows known devices. FIG. 7a is a photomultiplier tube mainly comprising an evacuated tube having a photocathode 701 with a transparent face plate, an anode 704, between them a multiplier section 702 with a defined number of individual dynodes 703. The photocathode 701 is designed to emit electrons into evacuated space 705, when radiation hits the photocathode. The photoelectrons are accelerated and focused to the first dynode. From left to right, the dynodes receive an increasingly positive voltage from an outside circuitry (not shown), thus accelerating electrons from left to right. Each individual dynode 703 is designed such that it generates, upon incidence of an electron, some secondary electrons drawn to the right side by the voltage of the next dynode to the right. Therefore, an amplifying effect is achieved, and finally a significant signal can be detected at anode 704. Due to the many individual parts to be assembled, the photomultiplier tube of FIG. 7a is costly. Besides that, it requires some external circuitry in order to apply the required voltages to the dynodes. It can suffer from instabilities in that electrons generated at the photocathode 701 might lead to charges at the inner walls of the outer housing 712, and, if the outer housing or parts of it are insulating, these charges would produce electric fields that might disturb the path of the electrons.
FIG. 7b shows a photomultiplier tube including a channel electron multiplier 711 (CEM), in which the CEM 711 is disposed within an outer housing 712. The outer 543-53.234EP-AP/wa housing 712 is evacuated and has on its left end the photocathode 701 with the transparent face plate. This device is bulky. The device has terminals 713, 714 for applying an accelerating voltage to the CEM 711. The applied voltage drops along a conductive path provided at the inside of the hollow, evacuated CEM 711. The multiplying section 711 in this embodiment is shown with a cone-shaped opening collecting electrons from the photocathode 701 and thereafter a helical portion in which electrons are accelerated by the electrical field caused by the voltage drop. Since along the inner wall of the CEM 711 a current continuously flows (currents ranging from some ten nanoamperes to some ten microamperes and voltages ranging from some hundred volts to some thousand volts), the CEM 711 is heated with a power corresponding to current and voltage drop. Since on the other hand the CEM 711 is disposed in an evacuated housing 712, there is no heat dissipation by convection or thermal conduction, so that the CEM 711 heats up until an equilibrium between heating and cooling by radiation is reached. This leads to electrical instabilities during the warm-up and cool-down phase in the case of high power dissipation. Furthermore, it limits a maximum current flow in the conductive path resulting in a very limited maximum anode current of the device and a small dynamic range.
Due to the bent structure of CEM 711 electrons repeatedly impinge on the walls and therefore cause secondary electrons, thus leading to an amplifying effect, so that at anode 704 a signal can be detected. Amplifications exceeding 10.sup.8 can be achieved with such a device.
FIG. 7c shows a detector known from EP-A-0 401 879. Within a monolithic ceramic body 721 a helical channel 722 is formed. The ends of the channel are terminated by a photocathode (not shown) on the one side and an anode portion on the other side. This device is complicated to manufacture, because forming a helical channel within the monolithic ceramic body and the generation of a conductive or semiconductive layer on the inner wall of the channel requires complex manufacturing techniques.
FIG. 7d shows an electron multiplier known from U.S. Pat. No. 3,243,628. It comprises a tubular body 731 coated at its inside with a resistive secondary emissive means 732.
FIG. 7e shows a tubular photocell known from U.S. Pat. No. 3,634,690. Here, a cathode 701 and an anode 704 are attached to the ends in lengthwise direction of a tube.